The subject matter disclosed herein relates to fire alarm systems and, more particularly, to a pneumatic pressure detector for a fire alarm system, as well as a method of insulating switches of the pneumatic pressure detector.
Fire alarm systems are employed to detect an overheat condition (e.g., fire) in a wide number of applications in many industries. For example, it is important to detect overheat conditions on aircraft or commercial vehicles. One approach is a pneumatic pressure detector that is part of a system that uses a gas that expands when heated. Upon heating, the gas actuates an associated deformable diaphragm, as well as any other type of switch, to close an electrical switch (e.g., fire alarm switch) to indicate an alarm condition. An integrity switch, or fault switch, also utilizes a deformable diaphragm. The integrity switch is electrically closed under normal operation, but will electrically open if the pneumatic pressure falls below a calibrated pressure. The fire alarm switch and the integrity switch are located, sealed and insulated within a housing.
Aerospace fire resistance standards ISO 2685 and AC 20-135 require that the housing pass a 2000° F. flame test for at least five minutes. The tests require that the housing containing the switches be located directly in the flame for the entire test, and that the pneumatic fire detector must operate as intended during this time. A challenge during the test is to protect the two pressure switches so that they are not exposed to the full heat load of the test. Switches exposed to too much heat during the test can result in the pressure setting dropping significantly, resulting in the pneumatic fire detector failing to either indicate the fire has been removed or the integrity pressure switch failing to indicate a severed sensing element.
Typically, the switches are potted in the housing in a manner to protect them from the full heat load of the 2000° F. flame. The potting material is put into the housing and cured at room temperature. During the test, it is possible that the viscosity of the potting material can change allowing the potting material to move and become reoriented within the housing. If this happens, the potting material can put excessive stresses on the switches and the pressure tubes attached to the switches as it cools when it is removed from the fire. These undue stresses may cause some type of failure or leak to occur during the cooling process resulting in a non-functioning pneumatic fire detector.
It should be noted that various potting materials are available for use, some of which are fire resistant, and others which can withstand extreme temperatures. However, under the full heat load of the five minute test at 2000° F., they all, to some degree, can experience a dimensional change due to thermal expansion and some also can outgas substances which can have detrimental material compatibility issues. It would also be possible that as the potting material expands during the test, the switches themselves could become reoriented causing them to come in contact with the metal housing and creating a dielectric failure. Another possibility is that as the potting material cools when it is removed from the fire the stress or force caused by the potting material's thermal contraction process could crack the interfacing pressure tubes. This is particularly true if the pressure tube material has been sensitized due to material compatibility issues.